1. Field of the Invention
This invention relates to a method synthesizing diamond in the vapor phase, by utilizing plasma and more particularly, it is concerned with a vapor phase synthesis of granular diamond suitable for use as an abrasive or grinding grains.
2. Description of the Prior Art
A vapor phase method of synthesizing diamond according to the prior art, by using plasma will now be illustrated by FIG. 2. Referring to FIG. 2, a in quartz tube 1 is arranged in microwave producing apparatus 5 and in this quartz tube 1 is arranged a support base 8 on which a plate-shaped substrate 7 is placed. As indicated by arrows, a mixed gas (raw material gas) of a volatile organic compound and hydrogen is subjected to microwave to form a plasma and diamond is deposited on the above described plate-shaped substrate 7 arranged in the plasma space.
However, the diamond synthesized in this way is film-shaped and can hardly be applied to articles which require granular shapes, for example, grinding grains. As is well known in the art, the vapor growth of granular diamonds is carried out by a method comprising dispersing and arranging substrate grains of diamond or silicon on the support base 8 instead of the plate-shaped substrate 7 as shown in FIG. 2 in which granular diamonds are grown from only the substrate grains. In this method, however, the number of the substrate grains to be dispersed and arranged is very small, that is, the number of granular diamonds obtained by one batch is so small that the production cost is increased and mass-production is difficult.
The reason why the number of the substrate grains to be arranged is small is that there is no way but to arrange them in two dimensions, that is, on only a certain plane. If the substrate grains can be arranged in three dimensions, that is, can be filled in a vessel, the number of the substrate grains is markedly increased. When the substrate grains are merely filled therein, however, there are small voids among the substrate grains, so a raw material gas does not form a plasma even if irradiated by microwaves. In order to render the raw material gas in the form of a plasma and thus deposit diamond on the substrate grains, it is required to provide voids at least in certain proportions among the substrate grains. Furthermore, it is apparent that even if a plasma is formed, the thus formed diamond is not in granular form.
Introduction of spaces or voids into among the substrate grains has, for example, been carried out by a method comprising arranging a vibration plate at the lower end of a packed layer of substrate grains and vibrating the vibration plate to disperse and float the substrate grains, as disclosed in Japanese Patent Publication No. 57568/1987. According to this method, however, there are formed both zones where the grains exist in a high concentration and zones where no grains exist and it is difficult to maintain a large amount of grains well dispersed therein.
In addition, there has been proposed a process for producing composite powdered grains, comprising feeding a mixed gas of CH.sub.4 and H.sub.2 under an excited state by a physical or chemical method and bringing it into contact with a powdered substrate in a CVD reaction apparatus. According to this process, however, it is also difficult to obtain a large amount of composite powdered grains in good quality.